use renameat2() if available to fix race condition in QFile::rename()

Signed-off-by: Ivailo Monev <xakepa10@gmail.com>

CMakeLists.txt

@@ -461,6 +461,7 @@ katie_check_function(pipe2 "unistd.h")
 katie_check_function(accept4 "sys/socket.h")
 katie_check_function(paccept "sys/socket.h")
 katie_check_function(getdomainname "unistd.h")
+katie_check_function(renameat2 "stdio.h")
 katie_check_struct(tm tm_gmtoff "time.h")
 katie_check_struct(tm tm_zone "time.h")
 katie_check_struct(sockaddr_ll sll_addr "netpacket/packet.h")

src/core/io/qfile.cpp

@@ -566,6 +566,7 @@ QFile::rename(const QString &newName)
         qWarning("QFile::rename: Empty or null file name");
         return false;
     }
+#ifndef QT_HAVE_RENAMEAT2
     if (QFile(newName).exists()) {
         // ### Race condition. If a file is moved in after this, it /will/ be
         // overwritten. On Unix, the proper solution is to use hardlinks:
@@ -573,6 +574,7 @@ QFile::rename(const QString &newName)
         d->setError(QFile::RenameError, tr("Destination file exists"));
         return false;
     }
+#endif
     unsetError();
     close();
     if(error() == QFile::NoError) {

src/core/io/qfilesystemengine_unix.cpp

@@ -387,8 +387,13 @@ bool QFileSystemEngine::renameFile(const QFileSystemEntry &source, const QFileSy
 {
     const QByteArray spath = source.nativeFilePath();
     const QByteArray tpath = target.nativeFilePath();
+#ifdef QT_HAVE_RENAMEAT2
+    if (::renameat2(AT_FDCWD, spath.constData(), AT_FDCWD, tpath.constData(), RENAME_NOREPLACE) == 0)
+        return true;
+#else
     if (::rename(spath.constData(), tpath.constData()) == 0)
         return true;
+#endif
     *error = errno;
     return false;
 }